Field of the Invention
The present invention relates to a signal processing apparatus to be used for an image reading apparatus, e.g., a scanner, which is configured to read an image of a document (hereinafter referred to as “document image”).
Description of the Related Art
An image reading apparatus is configured to receive, with a line sensor, reflected light of light that has been radiated to a document and generate an analog image signal representing a document image by performing photoelectric conversion of the received reflected light. The analog image signal is converted into a digital image signal by a sample-and-hold circuit and an AD converter. Analog signal processing circuits such as the sample-and-hold circuit and the AD converter are integrated to be prepared as a signal processing apparatus for conducting front-end processing. In addition to the sample-and-hold circuit and the AD converter, the signal processing apparatus includes, for example, a gain amplifier, a DA converter for offset adjustment, and a timing generator configured to generate a control signal for controlling an operation timing inside the signal processing apparatus.
In many cases, the signal processing apparatus is placed on a board having a line sensor mounted thereon in proximity to the line sensor. The digital image signal is transmitted from the signal processing apparatus to an apparatus in a subsequent stage (for example, apparatus for controlling an image reading apparatus) as a low-voltage differential signal (LVDS). In U.S. Pat. No. 8,081,100, there is disclosed an image reading apparatus configured to convert an analog image signal of three RGB colors output from the line sensor into five pairs of image LVDS signals for transmission. This image reading apparatus is configured to transmit, in synchronization with the image LVDS signals, a pair of clock signals (hereinafter referred to as “clock LVDS signal”) for frame synchronization. The clock LVDS signal is also an LVDS signal. In this image reading apparatus, for example, when the frequency of the analog image signal of each color is 40 MHz, the transfer rate of the image LVDS signal is 280 Mbps and the frequency of the clock LVDS signal is 40 MHz.
When the reading speed of a document image is increased, the frequency of an analog image signal to be output by the line sensor needs to be increased. In order to achieve the increased reading speed, the line sensor configured to read a color image transmits two analog image signals for each color to the signal processing apparatus. In this case, the transfer rate of the image LVDS signal to be output by the signal processing apparatus is also increased. For example, when the frequency of two analog image signals for each RGB color to be output by the line sensor is 40 MHz, the transfer rate for a pair of image LVDS signals is 560 Mbps and the frequency of the clock LVDS signal is 80 MHz.
The signal processing apparatus transmits an image LVDS signal and a clock LVDS signal through an inexpensive flexible flat cable. The flexible flat cable is not adequate for high-speed transmission, and as the cable length becomes longer, the degradation of a signal waveform becomes more severe. In order to maintain the waveform qualities of the image LVDS signal and the clock LVDS signal at an appropriate level, a dedicated cable optimized for transmission of LVDS signals is available. However, the dedicated cable is expensive and leads to an increased cost of the image reading apparatus, and thus is not a practical solution. It is an object of the present invention to provide a signal processing apparatus that enables higher speed processing of reading a document image.